It has been demonstrated on model systems that no difference between the radial distribution functions of molecules in isotopic mixtures can be observed by classical molecular dynamics simulations. In contrast, the Kirkwood–Buff integrals can yield information about the preferential solvation in these mixtures. The calculations of the Kirkwood–Buff integrals for the equimolar mixture of benzene and deuterobenzene have shown that in the case of positive excess Gibbs energy of the mixture, the isotopic molecules prefer to be surrounded by molecules of the same species. The extent of this preferential solvation is several orders of magnitude smaller than that observed in simple mixtures. 2004 Elsevier B.V. All rights reserved.